Erdheim-Chester disease (ECD) is a rare histiocytic disorder of unknown etiology that involves predominantly bone and viscera. Whether ECD represents a reactive or neoplastic process has been debated since its initial description. Herein, we report for the first time the cytogenetic findings of a case of ECD diagnosed at Mayo Clinic Rochester. The tumor occurred in the right tibia of a 35-year-old man and showed the balanced chromosomal translocation t(12;15;20)(q11;q24;p13.3), among other numeric chromosomal abnormalities. The lesion was positive for CD68 and negative for CD1a and S100. These findings support the idea that some cases of ECD are clonal neoplastic disorders of putative histiocytic differentiation. However, additional studies are warranted to confirm whether the chromosomal abnormalities found in this case represent recurrent cytogenetic events.